A Cross-Comparison of Reference Evapotranspiration Models using Field Observations

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Title: A Cross-Comparison of Reference Evapotranspiration Models using Field Observations
Author: Jones, Tiffanee N
Advisors: Fredrick H.M. Semazzi, Committee Member
Dr. Dev dutta S. Niyogi, Committee Chair
V.K. Saxena, Committee Member
Abstract: The natural systems of the earth are inter – dependant processes (and components of the coupled earth system) that rely heavily on environmental, meteorological, and hydrologic variables. The functions of the biosphere are influenced by many variables and contingencies, of which, the combined processes of evaporation and transpiration are extremely vital. Evapotranspiration (ET) estimation methodologies from empirical modeling to remote sensing have been evaluated on many occasions, by numerous organizations and individual researchers. One purpose of this research is to determine the ability of evapotranspiration formulations to replicate site specific field observations. In order to accomplish this objective, synthesis of evapotranspiration data for different environmental conditions was conducted over various landscapes. A secondary objective was to determine the impact of volumetric soil moisture on ET estimation schemes. To accomplish this, volumetric soil moisture observations were analyzed over each of the landscapes during the International H2O Project 2002 study period. A review of previous ET estimation research projects provided insight into the understood elements, and a basis for addressing the unknowns associated with ET estimation. The American Society of Civil Engineers (ASCE) has addressed the uncertainties associated with evapotranspiration on several occasions, but most specifically and extensively in Manual 70. Alternative ET estimation methodologies were considered and addressed in order to clarify the applicability of both models and remote sensing approaches. The primary focus of the project is centered on the International H2O Project 2002. By utilizing data from nine NCAR research stations over the Southern Great Plains region, ET estimation algorithms were tested against field observations. These tests showed the agreements of such equations as the ASCE Penman – Monteith with the ET values collected from each of the sites. The landscape varied from site to site and thus contributed to site specific model sensitivity. Supplementary analysis was conducted using Ameriflux data from the Bondville, Illinois station and North Carolina ECONet data from the Lake Wheeler, North Carolina station. The Bondville, Illinois data provided an extensive period of record and a viable data set for model evaluation. At this location the model performance was largely related to the location and study period. Meanwhile the Lake Wheeler, North Carolina data provided a glimpse of water stress impacts in the North Carolina Piedmont region. This data set consistently showed the highest predicted evapotranspiration from the model outputs and the highest observed air temperatures and solar radiation values. Finally, relative studies were conducted at the Soil Moisture Experiments 2003 (SMEX '03) in Tifton, Georgia and in the Central North Carolina piedmont. A case study of the localized climate variations due to land use change improved the understanding of the potential impact of land use change on individual meteorological variables (such as evapotranspiration). The SMEX '03 analysis demonstrates the impact of volumetric soil moisture on system water stress and the potential impact on ET estimation.
Date: 2005-03-01
Degree: MS
Discipline: Marine, Earth and Atmospheric Sciences
URI: http://www.lib.ncsu.edu/resolver/1840.16/888


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